1. Field of the Invention
The invention relates to detecting and counting surges, and, more particularly, to surge counter/detector apparatus and systems, such as, for example, low voltage metal oxide varistor (MOV) based surge suppressor systems. The invention also relates to a method for detecting and counting surge conditions on a power line.
2. Background Information
Surge arresters primarily protect the insulation breakdown of a conductor, such as an electric utility conductor. For example, an overhead transmission tower is employed to distribute electrical power from a generating plant to a substation, and then to end users, such as residential, commercial and industrial users. In a transmission tower, insulation is provided by the air space between conductors. A surge arrester prevents arcing between power line phases by diverting current caused by a transient overvoltage condition to the ground return path. The overvoltage condition may be attributed, for example, to lightning or capacitor bank switching. In an underground electrical system, where plastic or rubber insulation is employed, a surge arrester prevents rupturing of the insulation. Although the magnitude of the overvoltage is reduced, such reduced voltage may, nevertheless, damage downstream electrical equipment.
Surge suppressors, like surge arresters, are voltage clamping devices, which are employed to protect a load, such as, for example, appliances, computers and other electrical equipment, from surges. As such, a surge suppressor usually clamps the load voltage at a suitable voltage, which is less than the clamping voltage of the surge arrester. At the same time, the surge suppressor protects such electrical equipment from internal surge sources (e.g., after a circuit breaker panelboard), which result from other equipment (e.g., motor switching; operation of a switch to disconnect a load). The surge suppressor, thus, protects a load from both external sources (e.g., lightning voltage remnants) and internal disturbances (e.g, caused by other equipment). Surge suppressors typically include one or more capacitors to filter high frequency noise.
As defined by IEEE C62.41, there are three types of surges: (1) oscillatory surges or “ring waves” (e.g., a surge delivered to an electrical system excites natural resonant frequencies and, as result, has an oscillatory waveform less than about 1 kHz to 500 kHz, and may have different amplitudes); (2) high energy surges resulting from, for example, lightning, opening of a fuse, or power factor capacitor switching; and (3) a burst of very fast surges resulting from opening of air-gap switches or relays, which are typically represented by a 5 ns rise time and a 50 ns duration with various amplitudes. IEEE C62.41 also defines location categories with representative waveforms: (1) Category A: outlets and long branch circuits; (2) Category B: feeders, short branch circuits and distribution panels; and (3) Category C: outside and service entrance, such as run between a meter and a panel. For example, the lowest peak voltage and peak current is in Category A (e.g., 2 kV, 70 A), and the highest peak voltage and peak current is in Category C (e.g., 20 kV, 10 kA).
In order to count these diverse surges, a surge detector/counter must be able to work with various magnitudes and frequencies. At the same time, the surge detector/counter must be suitably fast in order to capture such surges. Furthermore, the surge detector/counter must count relatively high current surges coming from external sources and relatively low magnitude internal surges.
U.S. Pat. No. 4,338,648 discloses a surge counter in which the voltage across an arrester is rectified and stored in a capacitor, which acts as a peak detector. With this arrangement, fast rising transients are not captured and the counting circuit is exposed to high voltages.
U.S. Pat. No. 4,706,016 discloses a surge counter, which measures the voltage generated on a conductor ground return path. A capacitor stores the voltage, which is displayed by a counting circuit, which is exposed to high voltages.
U.S. Pat. No. 4,796,283 discloses a surge counter, which uses a current sensor on a ground return path and optically transfers the generated voltage to a counting circuit. A monostable generates a pulse, which increments the counter. The monostable pulse must be long enough in order to be detected by the display, but cannot be too long or, else, subsequent surges are not counted. Therefore, the timing is imprecise and is fixed for a particular counter.
Surge suppressors must protect the load or protected electrical device from lightning (e.g., voltage remnants from the surge arrester) and, also, from locally generated transients. Therefore, the corresponding counter must count both relatively high magnitude and relatively low magnitude transients.
U.S. Pat. No. 5,572,116 discloses a surge counter, which employs a spark gap as a sensor by measuring its light output. As such, the corresponding surge suppressor must have a spark gap. This is because the spark trigger voltage is dependent upon the rise time of the transient. If the surge suppressor is MOV based, then, for relatively fast rising transients, the MOV turns-on before the spark-gap, and, therefore, no sparking or light output is detected by the surge counting circuit. Hence, the circuit requires a spark gap to operate.
The known prior art does not prevent multiple false counting of various oscillatory type or “ring wave” surges.
There is a need for a surge counter/detector that counts a wide range of surges with different magnitudes and speeds.